1. Field of the Invention
The invention relates in general to Fizeau interferometers for optical testing.
2. Description of the Related Art
Phase-shift interferometry is an established method for measuring a variety of physical parameters ranging from the density of gasses to the displacement of solid objects. An interferometric wavefront sensor employing phase-shift interferometry typically consists of a temporally coherent light source that is split into two wavefronts, a reference and a test wavefront, that are later recombined after traveling different path lengths. The relative phase difference between the two wavefronts is manifested as a two-dimensional intensity pattern known as an interferogram. Phase-shift interferometers typically have an element in the path of the reference wavefront to introduce three or more known phase-steps or phase-shifts. By detecting the intensity pattern with a detector at each of the phase shifts, the phase distribution of the object wavefront can be quantitatively and rapidly calculated independently of the irradiance in the reference or object wavefronts.
Phase-shifting of the images can either be accomplished by sequentially introducing a phase-step (temporal phase-shifting), by splitting the beam into parallel channels for simultaneous phase-steps (parallel phase-shifting), or by introducing a high frequency spatial carrier onto the beam (spatial carrier phase-shifting). Parallel and spatial phase-shifting achieve data acquisition in times several orders of magnitude less than temporal phase-shifting, and thus offer significant vibration immunity. Several methods of parallel phase shifting have been disclosed in the prior art. Smythe and Moore (1983) and Koliopoulos (1993) describe a parallel phase shifting method where a series of conventional beam splitters and polarization optics are used to produce three or four phase shifted images onto as many cameras for simultaneous detection. A number of U.S. patents [U.S. Pat. No. 4,575,248 (1986), U.S. Pat. No. 5,589,938 (1996), U.S. Pat. No. 5,663,793 (1997), U.S. Pat. No. 5,777,741 (1998), U.S. Pat. No. 5,883,717 (1999)] disclose variations of this method where multiple cameras are used to detect multiple interferograms. Several prior-art publications (Barrientos, Kwon, Schwider) and patents (U.S. Pat. No. 6,304,330 and U.S. Pat. No. 6,552,808) describe methods to simultaneously image three or more interferograms onto a single sensor.
Tobiason et. al. (U.S. Pat. No. 6,850,329 and U.S. Pat. No. 6,847,457) and Brock et. al. in U.S. Pat. No. 7,230,717 describe spatial phase-shifting methods where a high frequency spatial pattern is encoded on the beam to effect simultaneous measurement without any significant division of the reference and test beams. These methods rely on orthogonally polarized reference and test beams and have the advantage of being true common-path arrangements. Distortions due to optical components such as zoom modules or beamsplitters do not affect the measurement accuracy.
Interferometers that have the test and reference surfaces located along the same optical axis (commonly known as Fizeau) offer advantages over other types of interferometers because they can be configured so that there are no elements between the test and reference surface. The Fizeau interferometer only requires one precision surface, which leads to greatly reduced manufacturing costs. Integrating a Fizeau interferometer with parallel or spatial phase-shifting techniques has proven somewhat difficult due to the need to encode opposite polarizations from reflections off nominally common optical path components and to a desire not to alter the surfaces or introduce an intra-cavity element. Sommargren (U.S. Pat. No. 4,606,638) teaches a method for absolute distance measurement that employs a Fizeau-type interferometer and uses a thin-film polarization reflection coating to separate the object and reference beams. However, the thin-film coating requires the incident and reflected wavefronts to be at a significant angle with respect to one another and only works over a narrow wavelength band. This significantly restricts the range at which the test optic can be placed, requiring the test and reference elements to be nearly in contact to avoid spatial separation between the wavefronts. In addition, it requires alteration of the cavity surfaces.
Millerd et al. (U.S. Pat. No. 7,057,738) describe a Fizeau interferometer that integrates a parallel phase-shifting sensor with a Fizeau interferometer. Tilt is used in the Fizeau interferometer cavity to either spatially separate the orthogonal polarization components for filtering on the receiving end, or to recombine orthogonal polarization components that were launched at different angles into the cavity. Introducing tilt in the Fizeau cavity in order to separate or combine the two polarization components has several undesirable consequences. First, the separate paths taken by the two polarizations can introduce aberrations into the measurement, particularly when using spherical reference optics. Second, it is necessary to spatially filter the beams at the imaging end to block unwanted polarizations. This reduces the number of tilt fringes that can be measured as well as the quality of the image.
In U.S. Pat. No. 4,872,755, Kuchel et al. proposed a method to provide orthogonally polarized reference and test beams in a Fizeau cavity without using tilt. By introducing an optical delay device in the measurement portion of the interferometer and judiciously selecting the coherence length of the light, the length of the delay path, and the length of the gap in the Fizeau cavity, two coherent test and reference beams as well as two incoherent beams are produced simultaneously. The delay device is used to vary the optical path difference between the two orthogonally polarized beams to ensure that they are still coherent with each other after the delay in the Fizeau cavity. Thus, the approach of Kuchel et al. requires fine adjustment of the length of the delay path, which is expensive and time consuming to implement. Kuchel (U.S. Pat. No. 6,717,680) also discloses an invention for eliminating stray reflections within an interferometer by modulating the Fizeau cavity with two external phase-shifters.
Finally, Brock et al. (U.S. Pat. No. 7,230,717) describe a spatial phase-shifting sensor integrated with a Fizeau interferometer using either a tilted beam arrangement with a long coherence source or an on-axis arrangement with a short coherence. While the combination of the spatial phase-shift sensor with either the tilted-beam Fizeau or delay-line Fizeau significantly extends the capability of each instrument, it does not overcome the inherent disadvantages of each. Therefore, there is still a need for a phase measurement system based on a Fizeau interferometer that does not suffer from the shortcomings of either the tilted beam or the short coherence approach.